llvm/lib/Transforms/Scalar/SimplifyCFGPass.cpp

//===- SimplifyCFGPass.cpp - CFG Simplification Pass ----------------------===//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This file implements dead code elimination and basic block merging, along
// with a collection of other peephole control flow optimizations.  For example:
//
//   * Removes basic blocks with no predecessors.
//   * Merges a basic block into its predecessor if there is only one and the
//     predecessor only has one successor.
//   * Eliminates PHI nodes for basic blocks with a single predecessor.
//   * Eliminates a basic block that only contains an unconditional branch.
//   * Changes invoke instructions to nounwind functions to be calls.
//   * Change things like "if (x) if (y)" into "if (x&y)".
//   * etc..
//
//===----------------------------------------------------------------------===//

#define DEBUG_TYPE "simplifycfg"
#include "llvm/Transforms/Scalar.h"
#include "llvm/Transforms/Utils/Local.h"
#include "llvm/Constants.h"
#include "llvm/Instructions.h"
#include "llvm/Module.h"
#include "llvm/Attributes.h"
#include "llvm/Support/CFG.h"
#include "llvm/Pass.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/Statistic.h"
using namespace llvm;

STATISTIC(NumSimpl, "Number of blocks simplified");

namespace {
  struct CFGSimplifyPass : public FunctionPass {
    static char ID; // Pass identification, replacement for typeid
    CFGSimplifyPass() : FunctionPass(&ID) {}

    virtual bool runOnFunction(Function &F);
  };
}

char CFGSimplifyPass::ID = 0;
static RegisterPass<CFGSimplifyPass> X("simplifycfg", "Simplify the CFG");

// Public interface to the CFGSimplification pass
FunctionPass *llvm::createCFGSimplificationPass() {
  return new CFGSimplifyPass();
}

/// ChangeToUnreachable - Insert an unreachable instruction before the specified
/// instruction, making it and the rest of the code in the block dead.
static void ChangeToUnreachable(Instruction *I) {
  BasicBlock *BB = I->getParent();
  // Loop over all of the successors, removing BB's entry from any PHI
  // nodes.
  for (succ_iterator SI = succ_begin(BB), SE = succ_end(BB); SI != SE; ++SI)
    (*SI)->removePredecessor(BB);
  
  new UnreachableInst(I->getContext(), I);
  
  // All instructions after this are dead.
  BasicBlock::iterator BBI = I, BBE = BB->end();
  while (BBI != BBE) {
    if (!BBI->use_empty())
      BBI->replaceAllUsesWith(UndefValue::get(BBI->getType()));
    BB->getInstList().erase(BBI++);
  }
}

/// ChangeToCall - Convert the specified invoke into a normal call.
static void ChangeToCall(InvokeInst *II) {
  BasicBlock *BB = II->getParent();
  SmallVector<Value*, 8> Args(II->op_begin()+3, II->op_end());
  CallInst *NewCall = CallInst::Create(II->getCalledValue(), Args.begin(),
                                       Args.end(), "", II);
  NewCall->takeName(II);
  NewCall->setCallingConv(II->getCallingConv());
  NewCall->setAttributes(II->getAttributes());
  II->replaceAllUsesWith(NewCall);

  // Follow the call by a branch to the normal destination.
  BranchInst::Create(II->getNormalDest(), II);

  // Update PHI nodes in the unwind destination
  II->getUnwindDest()->removePredecessor(BB);
  BB->getInstList().erase(II);
}

static bool MarkAliveBlocks(BasicBlock *BB,
                            SmallPtrSet<BasicBlock*, 128> &Reachable) {
  
  SmallVector<BasicBlock*, 128> Worklist;
  Worklist.push_back(BB);
  bool Changed = false;
  do {
    BB = Worklist.pop_back_val();
    
    if (!Reachable.insert(BB))
      continue;

    // Do a quick scan of the basic block, turning any obviously unreachable
    // instructions into LLVM unreachable insts.  The instruction combining pass
    // canonicalizes unreachable insts into stores to null or undef.
    for (BasicBlock::iterator BBI = BB->begin(), E = BB->end(); BBI != E;++BBI){
      if (CallInst *CI = dyn_cast<CallInst>(BBI)) {
        if (CI->doesNotReturn()) {
          // If we found a call to a no-return function, insert an unreachable
          // instruction after it.  Make sure there isn't *already* one there
          // though.
          ++BBI;
          if (!isa<UnreachableInst>(BBI)) {
            ChangeToUnreachable(BBI);
            Changed = true;
          }
          break;
        }
      }
      
      // Store to undef and store to null are undefined and used to signal that
      // they should be changed to unreachable by passes that can't modify the
      // CFG.
      if (StoreInst *SI = dyn_cast<StoreInst>(BBI)) {
        Value *Ptr = SI->getOperand(1);
        
        if (isa<UndefValue>(Ptr) ||
            (isa<ConstantPointerNull>(Ptr) &&
             SI->getPointerAddressSpace() == 0)) {
          ChangeToUnreachable(SI);
          Changed = true;
          break;
        }
      }
    }

    // Turn invokes that call 'nounwind' functions into ordinary calls.
    if (InvokeInst *II = dyn_cast<InvokeInst>(BB->getTerminator()))
      if (II->doesNotThrow()) {
        ChangeToCall(II);
        Changed = true;
      }

    Changed |= ConstantFoldTerminator(BB);
    for (succ_iterator SI = succ_begin(BB), SE = succ_end(BB); SI != SE; ++SI)
      Worklist.push_back(*SI);
  } while (!Worklist.empty());
  return Changed;
}

/// RemoveUnreachableBlocksFromFn - Remove blocks that are not reachable, even 
/// if they are in a dead cycle.  Return true if a change was made, false 
/// otherwise.
static bool RemoveUnreachableBlocksFromFn(Function &F) {
  SmallPtrSet<BasicBlock*, 128> Reachable;
  bool Changed = MarkAliveBlocks(F.begin(), Reachable);
  
  // If there are unreachable blocks in the CFG...
  if (Reachable.size() == F.size())
    return Changed;
  
  assert(Reachable.size() < F.size());
  NumSimpl += F.size()-Reachable.size();
  
  // Loop over all of the basic blocks that are not reachable, dropping all of
  // their internal references...
  for (Function::iterator BB = ++F.begin(), E = F.end(); BB != E; ++BB) {
    if (Reachable.count(BB))
      continue;
    
    for (succ_iterator SI = succ_begin(BB), SE = succ_end(BB); SI != SE; ++SI)
      if (Reachable.count(*SI))
        (*SI)->removePredecessor(BB);
    BB->dropAllReferences();
  }
  
  for (Function::iterator I = ++F.begin(); I != F.end();)
    if (!Reachable.count(I))
      I = F.getBasicBlockList().erase(I);
    else
      ++I;
  
  return true;
}

/// MergeEmptyReturnBlocks - If we have more than one empty (other than phi
/// node) return blocks, merge them together to promote recursive block merging.
static bool MergeEmptyReturnBlocks(Function &F) {
  bool Changed = false;
  
  BasicBlock *RetBlock = 0;
  
  // Scan all the blocks in the function, looking for empty return blocks.
  for (Function::iterator BBI = F.begin(), E = F.end(); BBI != E; ) {
    BasicBlock &BB = *BBI++;
    
    // Only look at return blocks.
    ReturnInst *Ret = dyn_cast<ReturnInst>(BB.getTerminator());
    if (Ret == 0) continue;
    
    // Only look at the block if it is empty or the only other thing in it is a
    // single PHI node that is the operand to the return.
    if (Ret != &BB.front()) {
      // Check for something else in the block.
      BasicBlock::iterator I = Ret;
      --I;
      if (!isa<PHINode>(I) || I != BB.begin() ||
          Ret->getNumOperands() == 0 ||
          Ret->getOperand(0) != I)
        continue;
    }
    
    // If this is the first returning block, remember it and keep going.
    if (RetBlock == 0) {
      RetBlock = &BB;
      continue;
    }
    
    // Otherwise, we found a duplicate return block.  Merge the two.
    Changed = true;
    
    // Case when there is no input to the return or when the returned values
    // agree is trivial.  Note that they can't agree if there are phis in the
    // blocks.
    if (Ret->getNumOperands() == 0 ||
        Ret->getOperand(0) == 
          cast<ReturnInst>(RetBlock->getTerminator())->getOperand(0)) {
      BB.replaceAllUsesWith(RetBlock);
      BB.eraseFromParent();
      continue;
    }
    
    // If the canonical return block has no PHI node, create one now.
    PHINode *RetBlockPHI = dyn_cast<PHINode>(RetBlock->begin());
    if (RetBlockPHI == 0) {
      Value *InVal = cast<ReturnInst>(RetBlock->begin())->getOperand(0);
      RetBlockPHI = PHINode::Create(Ret->getOperand(0)->getType(), "merge",
                                    &RetBlock->front());
      
      for (pred_iterator PI = pred_begin(RetBlock), E = pred_end(RetBlock);
           PI != E; ++PI)
        RetBlockPHI->addIncoming(InVal, *PI);
      RetBlock->getTerminator()->setOperand(0, RetBlockPHI);
    }
    
    // Turn BB into a block that just unconditionally branches to the return
    // block.  This handles the case when the two return blocks have a common
    // predecessor but that return different things.
    RetBlockPHI->addIncoming(Ret->getOperand(0), &BB);
    BB.getTerminator()->eraseFromParent();
    BranchInst::Create(RetBlock, &BB);
  }
  
  return Changed;
}

/// IterativeSimplifyCFG - Call SimplifyCFG on all the blocks in the function,
/// iterating until no more changes are made.
static bool IterativeSimplifyCFG(Function &F) {
  bool Changed = false;
  bool LocalChange = true;
  while (LocalChange) {
    LocalChange = false;
    
    // Loop over all of the basic blocks (except the first one) and remove them
    // if they are unneeded...
    //
    for (Function::iterator BBIt = ++F.begin(); BBIt != F.end(); ) {
      if (SimplifyCFG(BBIt++)) {
        LocalChange = true;
        ++NumSimpl;
      }
    }
    Changed |= LocalChange;
  }
  return Changed;
}

// It is possible that we may require multiple passes over the code to fully
// simplify the CFG.
//
bool CFGSimplifyPass::runOnFunction(Function &F) {
  bool EverChanged = RemoveUnreachableBlocksFromFn(F);
  EverChanged |= MergeEmptyReturnBlocks(F);
  EverChanged |= IterativeSimplifyCFG(F);
  
  // If neither pass changed anything, we're done.
  if (!EverChanged) return false;

  // IterativeSimplifyCFG can (rarely) make some loops dead.  If this happens,
  // RemoveUnreachableBlocksFromFn is needed to nuke them, which means we should
  // iterate between the two optimizations.  We structure the code like this to
  // avoid reruning IterativeSimplifyCFG if the second pass of 
  // RemoveUnreachableBlocksFromFn doesn't do anything.
  if (!RemoveUnreachableBlocksFromFn(F))
    return true;
  
  do {
    EverChanged = IterativeSimplifyCFG(F);
    EverChanged |= RemoveUnreachableBlocksFromFn(F);
  } while (EverChanged);
  
  return true;
}